1. Field of the Invention
This invention relates to processes for forming photovoltaic conductive features. More specifically, the invention relates to processes for forming photovoltaic conductive features from multiple inks, where at least one of the inks is capable of etching a passivation layer on a substrate. Such processes are useful in the production of photovoltaic cells.
2. Discussion of the Background Information
Improvement in solar cell efficiency can have a significant impact in the broad market adoption of solar cell technology. It is possible, for example, that a 0.2% cell efficiency improvement derived from improved performance of front grid electrodes for solar cells could lead to as much as $250M cost savings for the solar cell manufacturing market in 2010.
Major factors that inhibit solar cell efficiency include contact resistance and line resistance in the front grid electrodes. A portion of a typical solar cell is depicted schematically in FIG. 1. Under known screen printing methods for forming front grid electrodes, high contact resistance arises as a result of the processing conditions of the solar cell after a screen printing paste is applied to the passivation layer of a solar cell. The passivation layer often comprises silicon nitride. Typical screen printing pastes comprise more than 5 wt % micron-sized lead glass particles and on the order of 75 wt % micron-sized silver particles. The glass particles etch the silicon nitride passivation layer at the emitter/electrode interface at high processing temperatures (e.g., >800° C.). Etching of the silicon nitride passivation layer is necessary to achieve a satisfactory contact between the electrode and the emitter surface (e.g., the surface of an n-type semiconductor). At such high temperatures, however, the diffusion rate of silver in the silicon emitter layer of the solar cell is high and complete etching of the n-type semiconductor layer undesirably can result causing shunting in the cell, as shown in FIG. 1.
In addition, when screen printing is used to form the front grid electrodes, undesirably high line resistance may arise as a result of the amount of glass present in the bulk electrode layer. Because the glass particles are relatively large (in the micron size range), the glass particles have relatively low reactivity and do not sinter to form a dense body under the processing conditions described above. The presence of un-sintered glass particles in the bulk electrode layer leads to high porosity in that layer. Porosity in the bulk electrode layer may, in turn, lead to high line resistance in the bulk electrode layer.
While screen printing is an inexpensive and fast method for printing front grid electrodes, it also does not allow for the printing of narrow gridlines. In addition, the contact pressure associated with conventional screen printing methods can lead to solar cell breakage due to the fact that the cell must be mechanically handled. There is therefore a need for methods for printing front grid electrodes that afford narrow gridlines while minimizing cell breakage. Further, there is a need for methods for printing front grid electrodes that minimize the contact and line resistances.